Okay, so my husband calls them "Celts in Space", and yeah, it could be used as shorthand for that.
But this series is amazing. It has a wonderful protagonist, Aeron Aoibhell. It seamlessly blends magic and science, so it satisfies both my science fiction and fantasy cravings in one go. The story is deep, intricate and many-layed, and the characters are absolutely wonderful. And Patricia Kennealy-Morrison is one of my favourite authors and an incredibly cool lady in her own right.
Plus, Thomas Canty does the cover illustrations; he's also my favourite book cover illustrator.
I'm pretty sure the series is out of print (I have two copies of each book, all dog-eared), but it's well worth hunting them down.